Why dissolve edge acts differently
If you are looking for some kind of thumb rule you can generally say: In default mode with the option Dissolve Vertices enabled, it works like this: if the edge you would like to dissolve leaves one or more vertices with each only connected to two different edges, than these vertices (and the connected edges) will be dissolved as well and the bounding vertices will be connected with a new edge.
Example 1:

Example 2: